The World Health Organization (WHO) estimates that there are currently 34 million people infected with HIV worldwide and 1.3 million in North America.(1) Pneumonia remains a leading cause of morbidity and mortality in HIV-infected individuals, and it is well recognized that HIV infection increases susceptibility to a number of bacterial, mycobacterial, fungal and viral pulmonary infections.(2-5) Inhaled pollutants such as traffic-related air pollution may augment the risk of pulmonary infection in these already susceptible immunosuppressed individuals. Therefore we propose to study the effects of air pollutants on lung immunity and susceptibility to pulmonary infection in those with HIV. Our overall hypothesis is that traffic-related air pollution (TRP) is associated with an increased risk for pulmonary infections in HIV-infected individuals, and that these inhaled pollutants decrease alveolar macrophage phagocytosis. To test this hypothesis, we will perform a nested case-control study to evaluate TRP exposure as a predictor of pneumonia in HIV-infected persons (Specific Aim 1). With a second cohort of healthy HIV-positive and negative participants, we will perform a cross-sectional study to evaluate the effects of TRP on alveolar macrophage phagocytosis. We will obtain alveolar macrophages from each participant using bronchoscopy with bronchoalveolar lavage (BAL) (Specific Aim 2). A better understanding of the detrimental effects of inhaled pollutants on lung immunity and susceptibility to pneumonia could have immense implications in the prevention of pneumonia in immunosuppressed patients, and in policymaking to better protect our vulnerable populations with more effective regulation of harmful airborne pollutants.